Field of the Invention
The present invention relates to an endoscope objective optical system, and mainly to an objective optical system for a medical endoscope.
Description of the Related Art
As an endoscope intended for urinary organs, an endoscope (hereinafter, referred to as ‘endoscope for urinary organs’) having an inserting portion that is to be inserted transurethrally is available. In the endoscope for urinary organs, for letting the inserting portion to be insertable into urethra, it is indispensable to let an outer diameter of the inserting portion to be not more than 7 mm. Therefore, a diameter of the inserting portion of the endoscope for urinary organs is smaller than a diameter of an inserting portion of an endoscope for alimentary tract which is widely known for a medical examination of stomach or large intestine. The urinary organs are generally filled with urine. Therefore, an endoscope objective system used in the endoscope for urinary organs has an optical design envisaged for in-water observation.
In International Unexamined Patent Application Publication No. 2014/208373, an endoscope objective optical system with a wide angle of view at the time of in-water observation (hereinafter, referred to as ‘in-water angle of view’) has been disclosed. The endoscope objective optical system according to the abovementioned patent literature includes a first group having a negative refractive power, an aperture stop, and a second group having a positive refractive power. In this endoscope objective optical system of International Unexamined Patent Application Publication No. 2014/208373, the in-water angle of view is in a range of 105° to 164°.
Moreover, in International Unexamined Patent Application Publication No. 2014/208373, a medium of an observation space has been described. Here, the observation space is a space for observing urinary organs by the endoscope for urinary organs. In International Unexamined Patent Application Publication No. 2014/208373, it has been signified that the medium of the observation space in this case is a perfusion solution or urine of which water is the main constituent, and that it is reasonable to deem a refractive index of these media to be equivalent to the refractive index of water.
Furthermore, in International Unexamined Patent Application Publication No. 2014/208373, it has been pointed out that the in-water angle of view is narrowed with respect to an angle of view at the time of observation in air (hereinafter, referred to as ‘angle of view in air’). Moreover, in International Unexamined Patent Application Publication No. 2014/208373, a relationship of the angle of view in air and the in-water angle of view are shown as follows.
Angle of view in air180°160°140°120°In-water angle of view 97.2° 95.3° 89.7° 81.0°
The angle of view in air and in-water angle of view are calculated by letting a refractive index in water for a d-line to be 1.333 and a lens nearest to object in the endoscope objective optical system to be flat.
The abovementioned relationship signifies that even in an endoscope objective optical system with the angle of view in air of 120°, when this endoscope objective optical system is used in an endoscope for urinary bladder, the angle of view at the time of practical use, or in other words, at the time of in-water observation, is narrowed to 81°. In International Unexamined Patent Application Publication No. 2014/208373, a fact that a pathological lesion inside the entire urinary bladder cannot be explored efficiently even with an endoscope objective optical system having a wide angle of view in air has been indicated as an issue.
In Japanese Patent Application Laid-open Publication No. Hei 7-181377, an endoscope objective optical system in which it is possible to switch the angle of view or to reduce a variation in the angle of view has been disclosed. The variation in the angle of view is caused due to a machining accuracy and an assembly error of components. The endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. Hei 7-181377 includes a first group having a negative refractive power, an aperture stop, and a second group having a positive refractive power. A focal length is varied by moving some of the lenses.
In the endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. Hei 7-181377, the angle of view in air is 170° at a wide angle end. In an example 7 of Japanese Patent Application Laid-open Publication No. Hei 7-181377, a positive refractive power is imparted to a front-end surface of the optical system. Accordingly, in the example 7, the in-water angle of view is the widest. The in-water angle of view in the example 7 is 119°. In an endoscope objective optical system of which a front-end surface is flat, given that the in-water angle of view is less than 100° in general, the endoscope objective optical system according to the example 7 of Japanese Patent Application Laid-open Publication No. Hei 7-181377 can be said to be wide-angled even at the time of in-water observation.
In Japanese Patent Application Laid-open Publication No. 2006-3549, an image pickup unit for endoscope in which the variation in the angle of view is reduced has been disclosed. An endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. 2006/3549 includes a first group having a negative refractive power and a second group having a positive refractive power. The angle of view is varied by moving some of the lenses.
In Japanese Patent Application Laid-open Publication No. 2006-3549, the angle of view in air is in a range of 145° to 173°. When converted to the in-water angle of view, in an example in which a front-end surface is flat, the in-water angle of view is less than 100°. Moreover, an example 4 is an example in which the front-end surface has a positive refractive power and the in-water angle of view is the widest, but even in the example 4, the in-water angle of view is 105°.
In Japanese Patent Application Laid-open Publication No. 2002-14282, an endoscope objective optical system in which it is possible to switch the angle of view has been disclosed. The endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. 2002-14282 includes a first group having a negative refractive power, an aperture stop, and a second group having a positive refractive power. Moreover, zooming is carried out moving some of the lenses. The peculiarity of the endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. 200214282 is that at least two negative lenses are disposed in the first group specifically.
In all examples in Japanese Patent Application Laid-open Publication No. 2002-14282, a front-end surface is flat, and the angle of view in air is 138°. Therefore, the angle of view when converted to in-water angle of view is not more than about 89°.
In Japanese Patent Application Laid-open Publication No. 2012-47909, an endoscope objective optical system in which it is possible to switch the angle of view has been disclosed. In the endoscope objective optical system of Japanese Patent Application Laid-open Publication No. 2012-47909, the angle of view in air is not less than 180°. The endoscope objective optical system of Japanese Patent Application Laid-open Publication No. 2012-47909 includes a first group having a negative refractive power, a second group having a negative refractive power, an aperture stop, a third group having a positive refractive power, and a fourth group having a positive refractive power. Zooming is carried out by moving some of the lenses.
In Japanese Patent Application Laid-open Publication No. 2012-47909, the in-water observation has not been explained. However, in all examples, a front-end surface has a positive refractive power. Therefore, an arrangement of the endoscope objective optical system disclosed in Japanese Patent Application Laid-open Publication No. 2012-47909 is advantageous for widening the in-water angle of view. For such reasons, in the endoscope objective optical system of Japanese Patent Application Laid-open Publication No. 2012-47909, it is considered that a sufficiently large in-water angle of view can be secured.
In International Unexamined Patent Application Publication No. 2011/70897, an endoscope objective optical system with a super-wide angle of view has been disclosed. In the endoscope objective optical system disclosed in International Unexamined Patent Application Publication No. 2011-70897, the angle of view in air is not less than 180°. The endoscope objective optical system of International Unexamined Patent Application Publication No. 2011-70897 includes a first group having a negative refractive power, an aperture stop, and a second group having a positive refractive power.
In International Unexamined Patent Application Publication No. 2011-70897, the in-water observation has not been explained. However, in all examples, a front-end surface has a positive refractive power. Therefore, an arrangement of the endoscope objective optical system of International Unexamined Patent Application Publication No. 2011-70897 is advantageous for widening the in-water angle of view. For such reasons, in the endoscope objective optical system of International Unexamined Patent Application Publication No. 2011-70897, it is considered that a sufficiently large in-water angle of view can be secured.
In International Unexamined Patent Application Publication No. 2011/148822, an endoscope objective optical system with a super-wide angle of view has been disclosed. In the endoscope objective optical system disclosed in International Unexamined Patent Application Publication No. 2011/148822, the angle of view in air is not less than 180°. The endoscope objective optical system of International Unexamined Patent Application Publication No. 2011/148822 includes a first group having a negative refractive power, an aperture stop, and a second group having a positive refractive power.
In International Unexamined Patent Application Publication No. 2011/148822, the in-water observation has not been explained. However, in all examples, a front-end surface has a positive refractive power. Therefore, an arrangement of the endoscope objective optical system of International Unexamined Patent Application Publication No. 2011/148822 is advantageous for widening the in-water angle of view. For such reasons, in the endoscope objective optical system disclosed in International Unexamined Patent Application Publication No. 2011/148822, it is considered that a sufficiently large in-water angle of view can be secured.
In Japanese Patent Application Laid-open Publication No. Hei 5-288986 envisaging the in-water observation has been disclosed. However, even in examples (6 to 9, 17, and 18) in which the angle of view in air is the largest, the angle of view is 138.3°. This angle of view when converted to the in-water angle of view is 89°.